The present disclosure relates to a power conversion device that converts electric power between DC power and AC power of a plurality of phases.
Metal oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), and the like are known as switching elements to be used for power circuits such as an inverter circuit. The MOSFET and the IGBT have different electric characteristics because of differences in terms of physical properties and structures. In general, the power circuit such as an inverter circuit is structured by switching elements of the same type. Attempts have been made to achieve an electrically wide application range by structuring a circuit in which a plurality of types of switching element are mixed by taking advantage of the differences in terms of the electric characteristics of the switching elements. Japanese Patent Application Publication No. 2008-104282 from a source described below discloses an inverter circuit including arms in which an upper-stage switching element is the IGBT and a lower-stage switching element is the MOSFET.
This inverter circuit is driven, at the time of high load, through three-phase modulation control in which all three phases are subjected to switching control, and is driven, at the time of low load, through two-phase modulation control in which one phase out of the three phases is fixed and the two remaining phases are subjected to switching control. At the time of low load, a switching loss of the MOSFET is relatively smaller. Therefore, the two-phase modulation control is performed so that the duty factor of the lower-stage MOSFET is higher than the duty factor of the upper-stage IGBT. At the time of high load in which the current increase, a loss of the IGBT is relatively smaller. Therefore, the three-phase modulation control is performed so that the duty factor of the upper-stage IGBT is higher than the duty factor of the lower-stage MOSFET. In Japanese Patent Application Publication No. 2008-104282, through the control described above, the efficiency of the inverter circuit is improved over the entire operating range from the low load to the high load. In Japanese Patent Application Publication No. 2008-104282, there is used an inverter circuit including the IGBT at the upper stage and the MOSFET at the lower stage in order to selectively use the switching elements to be mainly used in an operating range in which the current is relatively small and an operating range in which the current is relatively large. Even though the MOSFET and the IGBT have different switching characteristics (for example, switching responses), there is no such description that the efficiency is improved and noise is reduced in the inverter circuit in consideration of, for example, a control mode in which the lower-stage MOSFET is operated at a switching frequency higher than that of the upper-stage IGBT.
When the upper stage and the lower stage of the arm of the inverter circuit are subjected to switching control complementarily, and when the switching element is controlled in an on state in accordance with the polarity of the generated alternating current, the current may flow in a forward direction with respect to a freewheeling diode. In this case, the current flows through both of the switching element and the freewheeling diode. Therefore, it is preferable to consider a loss of the freewheeling diode. In Japanese Patent Application Publication No. 2008-104282, respective freewheeling diodes having the same characteristics are connected in parallel to both of the IGBT and the MOSFET, and the losses of the freewheeling diodes are not taken into consideration. Those losses occur every time the switching is performed. Therefore, the losses are likely to increase particularly when. the switching frequency of the inverter circuit is increased.